The mechanism of cell fusion in Chlamydomonas reinhardi, a unicellular biflagellate green alga, will be investigated. Fusion between mating type plus (mt plus) and mt minus gametes is mediated by a mating structure, present in each mating type. The approach we propose to use includes an extension of ongoing research, which seeks to characterize individual molecules and their functional interactions upon activation for cell fusion. Specifically, we propose to purify the components of the mating structure and to investigate their biochemical and structural properties, emphasizing aspects which control the change from the quiescent to activated state. Activation in mt plus gametes involves the rapid polymerization of microfilaments and extension of the fertilization tubule. We are presently investigating the similarity these microfilaments bear to actin by heavy meromyosin labeling and detection of 3 methyl-histidine. In addition, we plan to examine the properties of microfilaments in imp-1, an mt plus mutant with a defective mating structure. We recently described the aggregation of intramembranous particles during activation of mt minus gametes. We propose to purify and characterize these fusion specific membrane particle aggregates. Furthermore, fluorescent labeling with selected lectins will be used to study changes in the outer surface of the mating structure upon activation. We also plan to examine whether the surface components and the membrane particles interact with one another. In addition, we will examine factors controlling membrane particle aggregation in normal cells and in mt minus mutants we propose to isolate. This examination should allow us to clarify some of the interactions controlling membrane particle movement.